Selective attention represents a key component of sensory processing. The vast amount of information impinging on the sensory epithelium is likely to routinely exceed the computational capacities of our brains. Several strategies have appeared in evolutionary history to reduce the computational demands of sensory processing by facilitating selection of the most salient environmental stimuli. Some of these strategies, like the elimination of redundant information at sub-cortical levels, seems to occur continuously and without conscious effort. Other strategies, like careful orientation of the visual or acoustic fovea towards salient features, depends on a closed sensory-motor loop and are often highly influenced by conscious thought and dynamically changing behavioral goals. Selective attention can focus limited computational resources on subsets of incoming stimuli, thereby accelerating or enhancing behavioral responses to relevant stimuli. Over the last twenty years, this psychological framework has been steadily extended to the neuronal level as neuroscientists have become increasingly aware that cortical, and even sub-cortical, neurons exhibit a surprising ability to dynamically reorganize in support of changing behavioral goals. Selective attention, at the neuronal level, represents another strategy for controlling the large volume of inflowing sensory information by attenuating irrelevant and/or amplifying relevant information. This proposal explores the effects of selective attention on neuronal selectivity in extrastriate visual cortex. Specifically, the experiments described here are aimed at characterizing the cortical circuitry responsible for dynamic changes in visual sensitivity and feature selectivity in extrastriate area V4 when selective attention is deployed during visual search. To investigate this question, we will record and characterize neuronal activity in awake primates performing attentionally demanding visual search tasks. In Specific Aim 1 we will investigate how feature-based attention can alter the preferred stimuli of V4 neurons in support of improved target detection. In Aim 2 we will investigate how the allocation of spatial attention is distributed in the face of increasing uncertainty about the exact spatial position of a search target. In both of these Aims we will combine multiple approaches, namely single neuron recordings and local field analysis to generate a more complete picture of how selective attention operates at the level of the cortical neuron and cortical microcircuit. PUBLIC HEALTH RELEVANCE The proposed research will advance our understanding of how "paying attention" improves performance on difficult tasks. In these experiments we will investigate how the visual system reconfigures when attention is deployed during visual search. In several increasingly common clinical conditions, for example autism and attention deficit hyperactivity disorder, the neuronal mechanisms underlying attentional deployment are thought to be substantially impaired, frequently leading directly to maladaptive behavior. Developing better models of attentional allocation and control will advance our understanding and the potential for treatment of these conditions.